In order to exhibit optimum reflective or retro-reflective properties, the surface of a marker should have a uniform structure. Hence, preferable manufacturing methods are those wherein finished markers exhibit the same reflective or retro-reflective properties at every point of their surface.
U.S. Pat. No. 3,964,820 describes a retro-reflecting element of generally globular configuration adapted to retro-reflect a ray of light back along its own incident path, when impinged by grazing light on a part-spherical portion of its surface, bulging above the surface of a support layer wherein such element is partially embedded. The element includes an essentially optically homogeneous body forming the part-spherical portion and provided, on its remaining surface, with a plurality of closely spaced inwardly concave and reflective part-spherical small bowls capable of sending back, by multiple reflections totaling 180 degrees, light rays impinging on the bulging part-spherical portion and refracted therethrough within said body towards any of the bowls.
U.S. Pat. No. 3,971,692 relates to a retro-reflective material that is manufactured by applying a layer of transparent glass spheres onto an adhesive covering formed on a substrate sheet, wherein the exposed portions of the spheres are coated with a reflective material such as aluminum, and the coated portions of the spheres are embedded in an adhesive coating of a component, wherein the substrate film is then removed.
U.S. Pat. No. 4,265,938 relates to retro-reflectors such as those used for road signs and vehicle number plates. Here, the surface of a metallic substrate is made retro-reflective by adhering a layer made of an organic polymer and glass spheres, and then guiding the substrate through a roller, wherein the glass spheres are covered with a crucible, such that the spheres create recesses in the metallic surface.